php 类中的静态属性

<?php
class ceshi{
    public static $map;
    public  function a(){
        self::$map =['a'];
    }
    public function b(){
        var_dump(self::$map);
    }
}
$ce = new ceshi();
$ce->a();
$ce->b();
echo '</br>';
$cee = new ceshi();
$cee->b();
echo '</br>';
class exercise{
    public  $map;
    public  function a(){
        $this->map =['a'];
    }
    public function b(){
        var_dump($this->map);
    }
}
$ce = new exercise();
$ce->a();
$ce->b();
echo '</br>';
$cee = new exercise();
$cee->b();
echo '</br>';